This invention relates to a chain tensioner for keeping constant the tension of mainly a chain or a toothed belt for driving camshafts.
Ordinarily, a chain transmission device comprising a sprocket mounted on one end of a crankshaft, sprockets mounted on ends of camshafts, and a chain trained around the sprockets to drive the camshafts further includes a chain guide pivotable toward a slack side of the chain, and a chain tensioner for applying an adjusting force to the chain guide to press the chain guide against the chain, thereby keeping constant the tension of the chain.
A chain tensioner used in such a chain transmission device is disclosed in JP Utility Model Publication 64-41755. This chain tensioner includes a body, a nut member having a rod at its front end and slidably inserted in the body, and a bolt member having a serration-shaped external thread formed on the outer periphery thereof and in threaded engagement with a serration-shaped internal thread formed on the inner periphery of the nut member. A return spring is mounted in the body to bias the nut member outwardly of the body. The serration-shaped threads have pressure flanks which receive a pushing force applied from the chain to the rod of the nut member through the chain guide, thereby preventing retraction of the nut member. While the pushing force is greater than the force of the return spring, the nut member retracts while rotating until the pushing force balances with the force of the return spring, thereby keeping constant the tension of the chain.
When the chain slackens, the nut member is pushed out while rotating under the force of the return spring, thereby re-tensioning the chain.
With this conventional chain tensioner, because the rod of the nut member has its front end in contact with the pivotable chain guide, when the chain guide pivots, turning moment acts on the nut member that tends to incline the nut member. Such turning moment directly acts on the portions of the internal and external threads that are in threaded engagement with each other.
Also, if, due e.g. to mounting errors, the front end of the rod is in contact with the chain guide at its point offset from its center, torque is applied to the nut member when the chain guide pivots.
Since the turning moment applied to the nut member directly acts on the serration-shaped threads, the serration-shaped threads may bite each other, thereby locking the nut member.
When torque is applied to the nut member, the nut member is moved axially while rotating. Thus, when such torque is applied while the tension of the chain is increasing, the resistance to vibration of the chain decreases. This impairs stable operation of the serration-shaped threads, thereby making it impossible to keep constant the tension of the chain.
If the chain vibrates with an amplitude larger than the axial play between the internal and external threads, because the nut member is prevented from retracting due to the contact between the pressure flanks of the internal and external threads, the nut member moves outwardly every time the chain slackens. Thus, the chain tends to be over-tensioned, so that its life is shortened.
An object of this invention is to provide a chain tensioner which operates stably at all times by preventing turning moment or torque applied from a pivotable chain guide from being transmitted to serration-shaped threads, and which can prevent over-tensioning of the chain.